Road bike tyre pressure

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For about a century, the answer to “what pressure should my road tyres be?” was as hard as you dare. Pump it to the number on the sidewall, because a harder tyre deforms less, and less deformation means less energy lost, and less energy lost means faster. That reasoning is correct. It is also incomplete, and the incomplete part is where all the interesting stuff lives.

Past a certain pressure, pumping your tyres harder makes you slower. Not less comfortable — slower. That’s the finding that rewrote this subject, and it has a name.

Two losses, pulling in opposite directions

There are two separate mechanisms taking energy out of your legs, and they respond to pressure in opposite ways. Almost every bad tyre-pressure take on the internet is someone who only knows about one of them.

Casing losses. As the wheel turns, the tyre flattens where it meets the road and springs back afterwards. Rubber and casing fabric are not perfectly elastic, so that flex-and-rebound cycle turns a little of your effort into heat. This happens thousands of times a minute. More pressure means less deformation means smaller casing losses. This is the mechanism the old advice knew about, and it’s why the old advice said pump it up.

Impedance losses.Real roads are not smooth. Every bump has to be negotiated: either the tyre absorbs it by deforming, or the tyre stays rigid and the bump lifts the bike, you, and everything you’re carrying. Lifting a rider is expensive, and the energy comes out of your legs; it also gets dissipated in your body and the frame rather than the tyre. More pressure means a stiffer tyre means the bike gets thrown around more means larger impedance losses.

Add them together and you don’t get a line, you get a U. Total rolling loss falls as you inflate — casing losses shrinking — and then, past some pressure, it starts rising again as impedance takes over. SILCA, which publishes the most accessible technical writing in this area, states the relationship directly: “Total Rolling Loss = Theoretical Crr Plus Impedance”.

Breakpoint pressure: the bottom of the U

The pressure at the bottom of that curve — where casing losses stop dominating and impedance takes over — is what SILCA calls the breakpoint pressure. Below it, you’re paying in casing flex. Above it, you’re paying to be bounced. At it, you are as fast as that tyre on that road can be.

SILCA publishes the history of this too, and credits it properly rather than claiming it: the initial drum testing was done by Tom Anhalt, Al Morrison and BicycleTireRollingResistance, and SILCA says it replicated the work in summer 2014 on a repaved 900-metre road course using the Chung Method. The key finding it reports from Anhalt’s field testing is that “real world data diverged somewhat dramatically from roller data at higher pressures” — which is exactly what you’d expect if a drum in a lab is smooth and a road is not. The lab was measuring casing losses beautifully and impedance not at all.

Two practical consequences follow, and they’re both published rather than ours:

The curve is not symmetrical, so err low.SILCA’s guidance is that it is “better to set your pressure a few psi below the BreakPoint Pressure than to have it a few PSI above”. The penalty for being under is gentle; the penalty for being over is steep, because impedance climbs fast. If you’re guessing, guess soft.

Rougher surface and narrower tyre both push the breakpoint down. The worse the road, the earlier impedance takes over. This is why the same rider on the same bike has a different correct pressure on fresh tarmac than on chipseal — and why cobbles are a different planet.

Why we won’t print a chart

Here’s where we part company with essentially every other page on this subject. You came here wanting a table: I weigh 75kg, I run 28mm tyres, tell me the number. We’re not going to, and we want to be precise about why, because “it depends” is usually a cop-out and this time it isn’t.

Look at what actually moves the breakpoint. SILCA lists the inputs in its own explanation of its calculator: “What surface are you riding on, what size are your tires, how much does your system weigh, what kind of bike are you riding; all of these play a factor in the optimal tire pressure.” And it adds the part the chart-makers ignore: “not all pavement, gravel, singletrack, or even wooden velodrome is created equal.”

System weight is not your weight.It’s you, plus the bike, plus bottles, plus tools, plus the bag, plus whatever you’re wearing. The chart asks for your body weight because that’s the number you know, not because it’s the number that matters.

Tyre width is not the number on the label.A tyre’s actual inflated width depends on the rim’s internal width, and a 28mm tyre on a modern wide rim measures meaningfully more than 28mm. Volume is what sets the physics, and the sidewall marking is a nominal claim about a different rim.

Surface is the largest single variable and the chart has no column for it. A chart cannot know whether your Tuesday loop is glass-smooth or forty years old. That variable moves the answer more than the difference between two riders twenty kilos apart — and it’s the one the chart silently drops.

Front and rear are not the same.Your weight distribution isn’t 50/50 and neither should your pressures be. The rear carries more, so it wants more. A single number for both wheels is already an approximation before you start.

So: a psi-per-kilo table takes a function of five or six variables, throws away four of them, and prints the result with two-decimal confidence. Everyone in this category ships one anyway, because it’s the thing readers ask for and it makes a great graphic. We can’t make one honestly, and we’d rather send you to a tool that asks you the questions than invent answers on your behalf. SILCA’s professional tyre pressure calculator takes the inputs a chart discards.

Who SILCA is, and what they sell

We’ve cited SILCA four times on this page, so you’re entitled to know their interest — that’s the deal on this site, and we apply it to everyone including sources we like.

SILCA makes a point of the relevant part themselves, in the calculator explainer: “We don’t make wheels or tires so we aren’t limited by our own products in the recommendations we provide.” That’s a real and unusually candid disclosure, and it’s the reason their recommendations can differ from a tyre brand’s. It is also not the whole picture: SILCA sells pumps, among other things, and a company whose business is inflation has a structural interest in tyre pressure being a subject you take seriously and measure precisely. That’s a much milder conflict than a tyre maker publishing tyre research, and we’d cite them again tomorrow — the mechanism they describe is physically checkable and they credit the outside researchers who did the original work rather than absorbing it. But “milder” isn’t “none”, and you should hold it while you read them.

What we can’t tell you

We have not measured a rolling resistance curve. We do not own a drum, a power meter protocol, or a 900-metre repaved test course, and we have not ridden the same loop at fifteen pressures with a stopwatch. Everything above is either a mechanism you can reason about from first principles or a claim SILCA published, quoted as theirs and linked so you can read the original.

The number you want — your psi, for your weight, on your road — depends on inputs only you have. That is not us being evasive. It is the actual finding, and every chart that pretends otherwise is deleting variables to make a table fit on a page.